This project investigates the role of genetic factors in primate vocal development. Current work focuses on the genetic and environmental factors influencing expression of the isolation call, a characteristic vocalization used for reestablishing contact between an infant and its caregiver, and between adult social partners. Previous reports have documented the importance of inherited factors in determining the detailed frequency vs. time pattern of the squirrel monkey isolation call, and that the infant isolation calls of a wide range of primate species are distinguishable by species-specific details but share an overall pattern of slowly modulated tonality. New findings this year are: (1) the isolation calls of pygmy marmosets over their first 8 weeks of life are distinguishable from those of mature individuals by virtue of the presence of numerous sounds with pronounced frequency modulation ("trills" and "phee notes"), in addition to the species-typical properties of repeated strings of more stereotyped notes with a characteristic frequency upsweep ("J-notes"); (2) young adults of the common marmoset, Callithrix jacchus, are robust sources of isolation calls when visually or acoustically separated from familiar colony members, producing long, loud whistles with individually distinctive structure which show significant and regular changes in certain parameters as time of separation increases; and (3) the squirrel monkeys of Costa Rica, which represent remnant populations with a long history of geographical isolation from the principal range of the genus in South America, demonstrate the same strong correlation between facial subtype and the acoustic characteristics of their species- typical isolation call, the Isolation Peep, as do South American monkeys.